This invention relates to fluid flowmeters, and more particularly to vortex shedding type meters wherein the vortices are detected by an optical Schlieren technique.
Small variations in fluid density, such as are caused by turbulence or vortices, result in local variations in the refractive index thus bending a light beam passing through such a vortex. The effect is commonly known as the Schlieren effect and is commonly employed in the examination of the flow of a fluid over a solid surface in wind tunnel equipment.
It is well known that when a bluff body is arranged in a stream, moving fluid vortices are shed from the body at a frequency corresponding to the fluid flow rate. Thus by detecting these vortices and measuring their frequency of propagation the fluid velocity may be determined. Optical detection of the vortices from their Schlieren effect measurement as light may be directed into and out of a fluid stream with minimal impedance to the fluid flow. Present Schlieren techniques, however, suffer from the problem that the detectors employed are insufficiently rigid to withstand vortex induced vibrations.